The present invention relates to a method of recovering oil from hydrocarbon saturated resources having low permeability and high porosity such as those comprising diatomite. Diatomite, also known as diatomaceous earth, is a sedimentary rock comprising the siliceous skeletal remains of single-celled algae called diatoms. The rock itself is unique, with porosities that can exceed 70% (double that of a comparable sandstone) but with extremely low permeability (as low as 1 md or less) that limit fluid flow through the rock. Some diatomite deposits have been saturated with hydrocarbons, creating unique oil and gas reservoirs holding large quantities of hydrocarbon resources. For example, California's diatomite resources located in the San Joaquin Basin are estimated to contain in excess of 19 billion barrels of original oil in place. As a result, there have been significant efforts to develop techniques to recover oil from this prolific resource over the past 4 decades, with only limited success.
Ex Situ Extraction Methods:
Due to its high porosity, diatomite resources exhibit exceptional fluid storage capacities, containing twice as much oil per unit volume of rock when compared to a traditional sandstone reservoir. Such a concentration of oil makes the rock particularly attractive as a mining target, as the ore value is significantly higher (double) than that of a traditional oil sands mining operation. Diatomite is also relatively soft, allowing easier, lower cost extraction of the material. As a result, there was initially a large interest in evaluating the potential for surface mining shallow diatomite oil resources, thus allowing one to bypass the permeability limitations of in situ oil recovery methods.
Prior mining processes all suffer from one or more of several defects or limitations that prevented them from being adopted as viable diatomite oil recovery methods. In particular, an economic method is required to properly separate the oil and rock derived from the chosen mining process.
Prior oil extraction methods from diatomaceous rock do not address the treatment of the diatomite as a slurry, but assume the rock is relatively dry, being extracted using traditional surface mining techniques. Notwithstanding, prior diatomite oil separation processes using extracting solvents fail to recover a sufficient amount of extracting solvent for reuse in the extracting process. Other processes fail to efficiently recover the extracting solvent. There are also problems associated with the presence of fines, including fines removal from product streams.
Still other processes produce emulsions, which are difficult and relatively expensive to handle. Yet other processes produce waste products which are likewise difficult to handle, while other processes use equipment which must be specially fabricated for use in the particular process. These and other defects or limitations are minimized if not eliminated by the present inventive method.
In Situ Extraction Methods:
As a result of the limitations of mining methods and associated ex situ oil recovery techniques, oil recovery processes were eventually adopted that are derivatives of more conventional in situ oil recovery methods, such as relying on the use of hydraulic fracturing to create flow paths by which the oil may flow from the rock. In cases where the diatomite resources are saturated with heavy oil, as is common in California, steam is injected to simultaneously lower the oil viscosity and fracture the rock. The reliance of current recovery methods on fracturing the formation limits recovery efficiencies since much of the rock is not contacted by the induced fracture systems and is bypassed, leaving the oil trapped within. Beyond having limited effectiveness at recovering oil from diatomite, current in situ processes are highly complex to manage and extremely inefficient.
In practice, steam fracturing recovery methods require large amounts of steam injection and take decades to recover only a small amount of the original oil in place. Environmental impacts are high, and the process efficiency is low. Following is a list of some of the highlighted weaknesses of current in situ recovery approaches:
High Carbon Footprint: As a result of burning fuel to generate steam, steam fracturing operations have some of the highest greenhouse gas emissions for oil recovery projects in the world.
Net Energy Consumption: Applying current in situ recovery methods may often require more energy to extract the oil that the energy content of the oil itself.
Subsidence and Well Failure: As a result of the high porosity and compressibility of diatomite rock, well failure rates can approach 50% per year if injection activities are not properly managed. Even with proper injection management, well failures are a common occurrence.
Surface Events: Eruptions of steam and water at surface are common due to the large injected volumes at shallow depths (sometimes less that 200 feet below the surface). These events have resulted in both environmental and safety threats.
Surface Impacts: Due to the low permeability of the rock, development requires extremely tight well spacing—as tight as ¼ acre spacing. Combined with a high intensity of surface infrastructure to support injection and production activities, a typical diatomite development results in significant long-term alterations to the natural environment.
Production costs are high, limiting the ability of operators to economically recover the resource.
It is apparent that an improved method of producing oil from low permeability high porosity resources such as diatomite is much desired.